Display apparatus and method for gamma correction

ABSTRACT

Image data of the respective colors are converted, by look-up tables in which γ correction data are stored, into gradation data that are white-balanced with intervention of the V-T characteristics of liquid crystal panels. The generated gradation data are supplied to D/A converters and the liquid crystal panels via liquid crystal driving circuits. Gamma correction data within dynamic ranges that are set for each display apparatus by measuring a luminance ratio and contrast ratios of the respective colors are written to the look-up tables in advance. As a result, all they correction data stored in the look-up tables can be used effectively. The invention is directed to such a method for generating data of look-up tables and an image display apparatus that performs gradation correction by using thus-formed look-up tables.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an image display apparatus. Inparticular, the invention relates to an image display apparatus capableof producing a proper display image by using look-up tables inperforming nonlinear processing such as a white balance adjustment andgamma correction in image display of displays, projectors, etc. as wellas to a data generation method used in storing correction data inlook-up tables.

[0003] 2. Description of the Related Art

[0004] The video signal of an image of usual TV broadcast is transmittedafter being subjected to γ correction so as to be suitable for thecurrent-luminance characteristic of the CRT. Therefore, in performingimage display based on such a video signal by using a display apparatusother than the CRT, it is necessary to perform gradation correction(hereinafter referred to as “γ correction”) that is suitable for theelectro-optical conversion characteristic of the display apparatus.

[0005]FIG. 1 is a four-quadrant diagram illustrating such γ correctionthat is performed for a liquid crystal panel as a display apparatus. Inthe case of a TV video signal, as shown in part (a) of FIG. 1, alow-level portion and a high-level portion of the original video signalis expanded and compressed, respectively, according to a ({fraction(1/2.2)})th-power curve before transmission.

[0006] Gamma correction for a video signal needs to be determined inconsideration of the V-T characteristic of the liquid crystal panel sothat the relationship between the original signal and the luminance onthe display screen becomes linear as shown in part (b) of FIG. 1 withintervention of the V-T characteristic of the liquid crystal panel shownin part (c) of FIG. 1. Part (d) of FIG. 1 shows an example of γcorrection characteristics.

[0007] A received video signal is subjected to such γ correction and acorrected signal is used as a drive signal of the liquid crystal panel.

[0008] With the above γ correction, signal processing can be performedso that the luminance of the liquid crystal panel properly reflects thelevel of an original video signal and hence the contrast of an originalimage can be reproduced correctly.

[0009] Also in the case of a color image, by performing such γcorrection for each of the three primary colors, the hue of an originalimage can be reproduced faithfully and color temperature setting and awhite balance adjustment can be attained by adjusting γ correctionvalues.

[0010] In the case of the liquid crystal display panel, the γ correctionassumes a nonlinear, S-shaped curve, which can be approximated by usinga polygonal line circuit in the case of an analog signal.

[0011] However, in recent years, for more faithful gradation expression,signal processing shown in FIG. 2, for example, has come to be employedin which digital signals of γ-corrected gradations are read out bysupplying 8-bit digital color video signals of R (red), G (green), and B(blue) (in the case of analog input signals, they are digitized inadvance by A/D converters) to look-up tables (memories) 11R, 11G, and11B in which γ correction data of the respective colors are stored inadvance, the read-out digital signals are converted into analog signalsby D/A converters 12R, 12G, and 12B, and the analog signals are suppliedas drive signals to liquid crystal panels 14R, 14G, and 14B of therespective colors via liquid crystal driving circuits 13R, 13G, and 13B.

[0012]FIG. 3 shows an example of γ correction performed by such a signalprocessing circuit. In FIG. 3, curves γR, γG, and γB of γ correctiondata that are stored in advance allow readout of output data (verticalaxis) for gradation data of 8 bits (0-256) of video signals that areinput to the look-up tables. As a result, for gradation levels oftransmission-side γ-corrected color signals, this γ correction enablesoutput of corrected signals with intervention of V-T drivecharacteristic curves VTr, VTg, and VTb of the liquid crystal panels.

[0013] A white balance adjustment is performed finally. Where onlyamplitude and offset adjustments are performed by the digital signalprocessing of the look-up tables, to reproduce white by γ-correctedsignals of the respective colors by compensating for optical variationsof the liquid crystal panels, the ranges of signals are restricted.Therefore, unnecessary signal ranges of the respective colors areeliminated. As shown in FIG. 3, usable γ-corrected output signal rangesof R, G, and B after the white balance adjustment are 20-100%, 8-80%,and 0-95%, respectively.

[0014] That is, only parts, in ranges Ra, Ga, and Ba, of the γcorrection curve data stored in the look-up tables are effectively usedas γ correction data and the other data are not used.

[0015] In particular, in the case of the transmission-type liquidcrystal panels, the contrast ratio of B is lower than the contrastratios of the other colors. For example, in general, the contrast ratiosof R and G on the liquid crystal panels are about 600:1 and about 500:1,for example. However, in the case of B, even in a case where the maximumdrive voltage is applied, the light attenuation factor is low and thereremains light that passes through the panel (leakage light). Therefore,the contrast ratio of B is as small as about 400:1. As a result, thescreen becomes bluish in a black range.

[0016] In view of the above, it is necessary to obtain proper whitebalance in a black-level-elevated state by adding the other colors toblue of minimum luminance. If the dynamic ranges of the other colors areso set equal to the dynamic range of B, as shown in FIG. 3 the upperlimit and the lower limit portions of drive voltages of R and G go outof the usable ranges after the white balance adjustment.

[0017] That is, the usable ranges after the white balance adjustment aredefined by the maximum values and minimum values that provide a constantluminance ratio of R, G, and B that is R:G:B=22:68:10, for example.Upper limits are determined by a white-side luminance ratio of R, G, andB. Black-side usable ranges of the respective colors are determined by ablack-side luminance ratio of R, G, and B, that is, (white-sideluminance ratio)×{1/(contrast of each color)}.

[0018] As described above, in the conventional γ correction method, notall of correction data of look-up tables that are stored in advance andconform to the V-T characteristics of the liquid crystal panels are usedeffectively; that is, the amount of data of the look-up tables that areused effectively is reduced. This results in a problem that thegradations of correction data unavoidably become coarse.

[0019] In the case of B, in particular, although the γ characteristicvaries steeply in a dark gradation range, output γ-corrected values inthis range have coarse gradations. This means a problem that a whitebalance adjustment cannot be performed closely in this dark range.

SUMMARY OF THE INVENTION

[0020] The present invention has been made in view of the aboveproblems. According to a first aspect of the invention, there isprovided an image display apparatus comprising look-up tables in which γcorrection data for color signals are stored, D/A converters forconverting color image data that are read out from the look-up tablesinto analog signals, respectively, driving means for generating drivesignals based on the analog signals, and optical modulating means fordisplaying a color image based on the drive signals, wherein γcorrection data that attain proper white balance in dynamic ranges thatcan be used for display are stored in the look-up tables, and whereinthe amplitudes and offsets of drive signals in such ranges as not to becorrected by the look-up tables are adjusted by signal transmissioncharacteristics of an analog system.

[0021] According to a second aspect of the invention, there is provideda gradation correction data generation method for an image displayapparatus comprising look-up tables in which γ correction data for colorsignals are stored, D/A converters for converting color image data thatare read out from the look-up tables into analog signals, respectively,driving means for generating drive signals based on the analog signals,and optical modulating means for displaying a color image based on thedrive signals, the gradation correction data generation methodcomprising the steps of, in a white balance adjustment and a γadjustment, setting dynamic ranges that can be used for display bymeasuring a luminance ratio and contrast ratios for the color signals,and performing a gain adjustment and an offset adjustment after D/Aconversion in such a manner that they are suitable for the dynamicranges; and re-calculating new γ correction data of the look-up tablesso that the new γ correction data have the dynamic ranges as fullranges, and substituting the new γ correction data for the previous γcorrection data of the look-up tables, whereby γ correction data withinthe dynamic ranges can be used effectively in the white balanceadjustment that is performed on digital data.

[0022] As described above, in the invention, to make such settings thatall the digital data stored in the look-up tables are used effectivelyin digitally performing such adjustments as γ correction and a whitebalance adjustment by the lookup tables, offsets and dynamic ranges arecompensated by subsequent analog circuits. Therefore, the adjustmentscan be performed by using finer luminance levels than in theconventional case even if memories having the same capacities as in theconventional case are used for storing γ correction data.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 illustrates a γ characteristic for a video signal;

[0024]FIG. 2 is a block diagram illustrating how γ correction isperformed by using look-up tables;

[0025]FIG. 3 shows an example of conventional γ correction;

[0026]FIG. 4 is a block diagram including γ correction look-up tablesand liquid crystal driving circuits according to the invention;

[0027]FIG. 5 is a flowchart showing how data to be stored in the look-uptables are generated;

[0028]FIG. 6 illustrates how various kinds of information are measuredfrom the screen of a display apparatus; and

[0029]FIG. 7 shows a signal conversion process according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030]FIG. 4 is a block diagram outlining how signal processing of γcorrection and a white balance adjustment are performed to obtain colordrive signals to be supplied to display sections.

[0031] It is assumed that color video input signals of red (R), green(G), and blue (B) that are supplied from a video source (not shown inFIG. 4) are image data that are digital signals of 8-bit gradation, forexample, obtained by A/D conversion.

[0032] The image data of the respective colors are supplied torespective look-up tables 21R, 21G, and 21B in which data to producewhite-balanced signals are stored. The look-up tables 21R, 21G, and 21Bconvert the image data into data having gradation levels that provideproper white balance with intervention of the V-T characteristics ofliquid crystal panels as the display sections.

[0033] The look-up tables 21R, 21G, and 21B are memories (e.g., EPROMS)that produce an output of 10 bits, for example. The data of the look-uptables 21R, 21G, and 21B can be rewritten externally, and rewritten dataare held by the look-up tables 21R, 21G, and 21B.

[0034] Data that are read out from the look-up tables 21R, 21G, and 21Bare supplied to D/A converters 22R, 22G, and 22B, where they areconverted into analog signals. The analog signals are supplied to theliquid crystal panels 24R, 24G, and 24B via liquid crystal drivingcircuits 23R, 23G, and 23B.

[0035] Where the display apparatus is a projector, the liquid crystalpanels 24R, 24G, and 24B modulate light beams emitted from a halogenlamp or the like in accordance with video signals and are usuallynormally-white type light valves in which when a voltage is applied to acertain pixel, the quantity of light passing through the pixel decreasesin accordance with the voltage value.

[0036] Each of the D/A converters 22R, 22G, and 22B is configured insuch a manner that the output step width of 1 bit varies when areference voltage supplied is varied by a control section (not shown).As a result, each of the D/A converters 22R, 22G, and 22B has a gainsetting means capable of converting the amplitude of a signal indicatedby input data (i.e., data that is input to the D/A converter) at aprescribed gain. Each of the liquid crystal driving circuits 23R, 23G,and 23B also has an offset setting means for varying the offset level bysuperimposing a DC component to a signal voltage value to be output.

[0037] Next, a description will be made of a process of writing data tothe look-up tables so that with the above-described signal processingblocks a white-balance-adjusted image is displayed by light beams thathave passed through the liquid crystal panels after execution of the γcorrection for each color.

[0038]FIG. 5 is a flowchart showing a method for writing data to look-uptables and a white balance adjustment method that are used in the imagedisplay apparatus according to the invention. FIG. 6 outlinesinstruments for measuring luminance, contrast, etc. of each displayedcolor in which the video display screen is the screen of a rearprojector, for example.

[0039] A measuring method is as follows. An image on a screen 1 a (videodisplay screen) of a liquid crystal projector (rear projector), forexample, as an apparatus 1 to be measured is measured with a luminancemeter 2 and a colorimeter 4 in combination with replaceable filters 5 ofR, G, and B. Resulting luminance data and color difference data of eachcolor are captured by a computer 3 that is composed of a CPU, a memory,a display section, peripheral devices, interfaces, etc.

[0040] In measuring luminance of R, G, or B, the filter 5 of R, G, or Bis placed in front of the incidence lens of the luminance meter 2. Thefilters 5 are not used in a measurement with the colorimeter 4.

[0041] Depending on the content of measurement, the computer 3 suppliesa reference measurement image signal (e.g., a flat pattern signal of R,G, B, or black) to the video input terminal of the liquid crystalprojector 1 and causes the liquid crystal projector 1 to display aprescribed image. As described later, y correction data that have beennewly calculated based on measurement results are written to the look-uptables of the liquid crystal projector 1.

[0042] A data generation method is as follows. First, at step S1, flatfield images of R, G, and B corresponding to maximum digital γcorrection values are displayed by using a signal source that isprovided in a digital gamma correction section (IC) and luminance ofeach color is measured.

[0043] At step S2, flat field images of R, G, and B corresponding tominimum digital γ correction values are displayed by using the signalsource that is provided in the digital gamma correction section andluminance of each color is measured. Contrast ratios of the respectivecolors are obtained by calculating ratios between the luminance valuesobtained at step S1 and those obtained at this step.

[0044] At step S3, data of standard V-T characteristics (curves) of therespective colors of the liquid crystal panels are captured by thecomputer 3. At step S4, ranges where the luminance ratio among the threecolors is kept constant, that is, white balance is maintained, aredetermined based on the above-calculated contrast ratios and thestandard V-T characteristic data. Offsets and gains for these usableranges (dynamic ranges) are calculated. Gains of the D/A converters 22R,22G, and 22B and offsets of the liquid crystal driving circuits 23R,23G, and 23B (see FIG. 4) are so set as to conform to the dynamic rangesthus calculated.

[0045] At step S5, contrast magnifications are calculated based on thestandard V-T characteristics and V-T characteristic data are convertedso that the contrast ratios of R, G, and B become identical.

[0046] At step S6, usable ranges of the look-up tables are determinedbased on the offsets and the gains of the respective colors andcalculations are performed so that the γ correction data in the usableranges are converted into new γ correction data that occupy the entiredynamic ranges of the respective look-up tables.

[0047] At step S7, the new γ correction data are supplied from thecomputer 3 to the look-up tables that are incorporated in the liquidcrystal projector 1 and data rewriting is performed. The dynamic rangechanging process is finished here.

[0048] Steps S10-S14 are white balance adjustment steps that are alsoexecuted in the prior art and that are executed in the invention afterrewriting of the data of the look-up tables.

[0049] At step S11, a white balance adjustment signal (e.g., a flatfield signal) of an IRE 30% level is supplied from the computer 3 to theliquid crystal projector 1. An image on the liquid crystal projector 1is measured with a colorimeter 4 and the offsets of the digital gammacorrection section are adjusted so that the white balance values haveprescribed values.

[0050] At step S12 a white balance adjustment signal (e.g., a flat fieldsignal) of an IRE 70% level is supplied from the computer 3 to theliquid crystal projector 1. An image on the liquid crystal projector 1is measured with a colorimeter 4 and the gains of the digital gammacorrection section are adjusted so that the white balance values haveprescribed values.

[0051] At step S13, it is judged whether the white balance of each ofimages displayed while white balance adjustment signals of IRE 30% and70% levels are supplied is within a prescribed error range. If the whitebalance is not within the prescribed error range, steps S11 and S12 areexecuted again. If the white balance is within the prescribed errorrange, the adjustment is finished and a transition is made to step S14.

[0052] If the γ correction data of the look-up tables of the liquidcrystal projector 1 have been rewritten by execution of steps S1-S9, allof the stored correction data are data that are usable in the entiredynamic ranges of input signals. The accuracy per bit of correction datacan be made higher than in the conventional case even if the memorycapacities of the look-up tables are the same.

[0053] The above-described block diagram of FIG. 4 shows the gradationcorrection driving circuits of the display apparatus according to theinvention in which the data of the look-up tables have been rewritten inthe above-described manner.

[0054] In the invention, in the white balance adjustment and the γadjustment, dynamic ranges that can be used for display are determinedfirst by measuring a luminance ratio and contrast ratios and gains andoffsets for analog signals obtained by D/A conversion are determinedthereafter so as to conform to the dynamic ranges.

[0055] In the case of FIG. 4, the gains and the offsets of therespective colors are adjusted by the D/A converters 22R, 22G, and 22Band the liquid crystal driving circuits 23R, 23G, and 23B, respectively.However, the invention is not limited to such a circuit configuration.

[0056]FIG. 7 shows examples of γ correction characteristics and V-Tcharacteristics of the respective colors that are used in a case wherethe gradation correction is performed by the circuit configuration ofFIG. 4.

[0057] In FIG. 7, an R input, a G input, and a B input of digitizedimage data are converted into prescribed image data according to curvesγR, γG, and γB of the γ correction data that are stored in the look-uptables 21R, 21G, and 21B, respectively. The resulting image data aregain-adjusted by the D/A converters 22R, 22G, and 22B and thenoffset-adjusted by the liquid crystal driving circuits 23R, 23G, and23B.

[0058] Gamma correction data to be written to the look-up tables 21R,21G, and 21B have been generated by the process of FIG. 5 as correctiondata all of which are within dynamic ranges of the respective colorsthat are necessary for attaining proper white balance. Outputs (drivesignals) produced as a result of the correction according to suchcorrection data are gain/offset-adjusted in the form of analog signalsand then supplied to the liquid crystal panels 24R, 24G, and 24B.

[0059] By driving the liquid crystal panels 24R, 24G, and 24B by usingsuch drive signals, images of the three colors that have beenwhite-balanced according to the V-T curves are projected onto the screenor the like.

[0060] Therefore, the usable output ranges after the white balanceadjustment are wider than that of being obtained by conventional γcorrection data that are stored in advance and the data can be usedeffectively in the ranges Ra, Ga, and Ba of the γ correction curves γR,γG, and γB.

[0061] As described above, in the invention, dynamic ranges of therespective colors that can be used for display are determined first foreach display apparatus by measuring a luminance ratio, contrast ratiosof the respective colors, etc. with a luminance meter and a colorimeterand then the gain adjustment of the D/A converters and the offsetadjustment of the liquid crystal driving circuits are performed in sucha manner as to be suitable for the thus-determined dynamic ranges. Sinceγ correction data within the thus-determined dynamic ranges are writtento the look-up tables, all the γ correction data can be usedeffectively, which enable a more correct white balance adjustment by anincreased data amount.

[0062] In particular, the invention makes it possible to output a γcorrection value of B with high accuracy (i.e., with a larger number ofbits) in a black balance adjustment in which images are displayed at lowluminance.

[0063] As described above, in the invention, in performing image displayby using devices having nonlinear signal-luminance characteristics as inthe case of liquid crystal panels or the like, dynamic ranges correctedby the γ adjustment and the white balance adjustment on digital data areset by the gain adjustment and the offset-adjustment by the analogcircuits. Therefore, where the correction on digital data is performedby look-up tables, all the correction data can be used effectively. Thisprovides an advantage that the capacities of memories for the correctioncan be made smaller than in the conventional case.

[0064] Where look-up tables having the same memory capacities as in theconventional case are used, the bit accuracy of necessary colorcorrection values can be increased and the correction characteristicsare made even smoother. This provides an advantage that color imbalancethat is prone to occur particularly in a dark range can be compensatedfor with increased accuracy.

What is claimed is:
 1. An image display apparatus comprising: look-uptables in which γ correction data for converting color signals intowhite-balanced digital color image data within dynamic ranges ofrespective colors that are usable for display are stored; and D/Aconverters for converting the digital color image data into analogsignals, respectively, while performing a gain adjustment; driving meansfor generating drive signals while adjusting offsets of the analogsignals; and optical modulating means for displaying a color image basedon the drive signals.
 2. The image display apparatus according to claim1 , wherein the color signals are color signals of three primary colorsof red, green, and blue, and wherein the optical modulating means isthree liquid crystal panels corresponding to red, green, and blue. 3.The image display apparatus according to claim 2 , wherein the liquidcrystal panels are of a normally-white, transmission type.
 4. Agradation correction data generation method for an image displayapparatus comprising look-up tables in which γ correction data for colorsignals are stored, D/A converters for converting color image data thatare read out from the look-up tables into analog signals, respectively,driving means for generating drive signals based on the analog signals,and optical modulating means for displaying a color image based on thedrive signals, the gradation correction data generation methodcomprising the steps of: in a white balance adjustment and a γadjustment, setting dynamic ranges that can be used for display bymeasuring a luminance ratio and contrast ratios for the color signals,and performing a gain adjustment and an offset adjustment after D/Aconversion in such a manner that they are suitable for the dynamicranges; and re-calculating new γ correction data of the look-up tablesso that the new γ correction data have the dynamic ranges as fullranges, and substituting the new γ correction data for the previous γcorrection data of the look-up tables, whereby γ correction data withinthe dynamic ranges can be used effectively in the white balanceadjustment that is performed on digital data.
 5. The gradationcorrection data generation method according to claim 4 , wherein thecolor signals are color signals of three primary colors of red, green,and blue, and wherein the optical modulating means is three liquidcrystal panels corresponding to red, green, and blue.